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Observational Approaches to Understanding Dark Energy

Observational Approaches to Understanding Dark Energy. Yun Wang Univ. of Oklahoma Lepton and Photon 2007 Daegu, Korea August 16, 2007. beware of the dark side … Master Yoda. Outline. How do we know dark energy exists? Observational methods for dark energy search

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Observational Approaches to Understanding Dark Energy

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  1. Observational Approaches to Understanding Dark Energy Yun Wang Univ. of Oklahoma Lepton and Photon 2007 Daegu, Korea August 16, 2007

  2. beware of the dark side … Master Yoda Yun Wang, LP07, 8/16/2007

  3. Outline • How do we know dark energy exists? • Observational methods for dark energy search • Future prospects: recommendations by • the Dark Energy Task Force • ESA-ESO Working Group on Fundamental Cosmology Yun Wang, LP07, 8/16/2007

  4. How do we know there is dark energy?We infer its existence via its influence on the expansion history of the universe. Yun Wang, LP07, 8/16/2007

  5. First Evidence for Dark Energy in the Hubble Diagrams of Supernovae [dL(z)] (Schmidt et al. 1998, Perlmutter et al. 1999) Yun Wang, LP07, 8/16/2007

  6. Alternative Analysis of First Evidence Flux-averaged and combined data of 92 SNe Ia from Schmidt et al. (1998) and Perlmutter et al. (1999).[Wang 2000b, ApJ ] Deceleration parameter q0=m/2- Data favor q0 <0: cosmic acceleration Yun Wang, LP07, 8/16/2007

  7. Wang & Tegmark 2005 Yun Wang, LP07, 8/16/2007

  8. w(z) = w0+wa(1-a) 1+z = 1/a z: cosmological redshift a: cosmic scale factor WMAP3 +182 SNe Ia (Riess et al. 2007, inc SNLS and nearby SNe) +SDSS BAO (Wang & Mukherjee 2007) Yun Wang, LP07, 8/16/2007

  9. Model-independent constraints on dark energy(as proposed by Wang & Garnavich 2001) Wang & Mukherjee (2007) Yun Wang, LP07, 8/16/2007

  10. Wang & Mukherjee (2007) [See Wang & Tegmark (2005) for the method to derive uncorrelated estimate of H(z) using SNe.] H(z) = [da/dt]/a Yun Wang, LP07, 8/16/2007

  11. Some Candidates for Dark Energy cosmological constant(Einstein 1917) quintessence(Freese, Adams, Frieman, Mottola 1987; Linde 1987; Peebles & Ratra 1988; Frieman et al. 1995; Caldwell, Dave, & Steinhardt 1998; Dodelson, Kaplinghat, & Stewart 2000) k-essence:(Armendariz-Picon, Mukhanov, & Steinhardt 2000) Modified Gravity Vacuum Metamorphosis(Parker & Raval1999) Modified Friedmann Equation (Freese & Lewis 2002) Phantom DE from Quantum Effects(Onemli & Woodard 2004) Backreaction of Cosmo. Perturbations (Kolb, Matarrese, & Riotto 2005) Yun Wang, LP07, 8/16/2007

  12. How We Probe Dark Energy • Cosmic expansion history H(z) or DE density X(z) tells us whether DE is a cosmological constant H2(z) = 8 G[m(z) + r(z) +X(z)]/3  k/a2 • Growth history of cosmic large scale structure G(z) tells us whether general relativity is modified Yun Wang, LP07, 8/16/2007

  13. Observational Methods for Dark Energy Search • SNe Ia (Standard Candles): method through which DE has been discovered; independent of clustering of matter, probes H(z) • Baryon Acoustic Oscillations (Standard Ruler): calibrated by CMB, probes H(z) • Weak Lensing Tomography and Cross-Correlation Cosmography: probes clusteringof matter G(z), and H(z) • Galaxy Cluster Statistics: probes H(z) Yun Wang, LP07, 8/16/2007

  14. Supernovae as Standard Candles The SNe Ia lightcurves (left) are very different from that of SNe II (below). Measuring the apparent peak brightness and the redshift of SNe Ia gives dL(z), hence H(z) Yun Wang, LP07, 8/16/2007

  15. Theoretical understanding of SNe Ia Binary  C/O white dwarf at the Chandrasekher limit (~ 1.4 MSun)  explosion  radioactive decay of 56Ni and 56Co: observed brightness • explosion: carbon burning begins as a turbulent deflagration, then makes a transition to a supersonic detonation • earlier transition: cooler explosion  less 56Ni produced: dimmer SN Ia lower opacity faster decline of the SN brightness Wheeler 2002 (resource letter) Yun Wang, LP07, 8/16/2007

  16. Calibration of SNe Ia Phillips 1993 Riess, Press, & Kirshner 1995 Brighter SNe Ia decline more slowly • make a correction to the brightness based on the decline rate. 26 SNe Ia with Bmax-Vmax  0.20 from the Calan/Tololo sample [Hamuy et al. 1996, AJ, 112, 2398] Yun Wang, LP07, 8/16/2007

  17. Getting the most distant SNe Ia: critical for measuring the evolution in dark energy density: Wang & Lovelave (2001) Yun Wang, LP07, 8/16/2007

  18. SNe Ia as Cosmological Standard Candles Systematic effects: dust: can be constrained using multi-color data. (Riess et al. 1998; Perlmutter et al. 1999) gray dust:constrained by the cosmic far infrared background. (Aguirre & Haiman 2000) gravitational lensing: its effects can be reduced by flux-averaging. (Wang 2000; Wang, Holz, & Munshi 2002) SN Ia evolution (progenitor population drift): Once we obtain a large number of SNe Ia at high z (z > 1), we can disregard SN Ia events that have no counterparts at high z, and only compare like with like. (Branch et al., astro-ph/0109070) Yun Wang, LP07, 8/16/2007

  19. Baryon acoustic oscillations as a standard ruler Blake & Glazebrook 2003 Seo & Eisenstein 2003 Comparing the observed acoustic scale with its expected values gives us H(z) [radial direction] and DA(z) [transverse direction] Yun Wang, LP07, 8/16/2007

  20. Detection of BAO in the SDSS data[Eisenstein et al. 2005] Yun Wang, LP07, 8/16/2007

  21. DE eq. of state w(z)=w0+wz Wang 2006 Yun Wang, LP07, 8/16/2007

  22. BAO systematic effects • Galaxy clustering bias • Redshift space distortions • Nonlinear gravitational clustering Yun Wang, LP07, 8/16/2007

  23. Weak Lensing Tomography and Cross-Correlation Cosmography Yun Wang, LP07, 8/16/2007

  24. Weak Lensing Tomography: compare observed cosmic shear correlations with theoretical/numerical predictions to measure cosmic large scale structure growth history G(z) and H(z) [Wittman et al. 2000] • WL Cross-Correlation Cosmography measure the relative shear signals of galaxies at different distances for the same foreground mass distribution: gives distance ratios dA(zi)/dA(zj) that can be used to obtain cosmic expansion history H(z) [Jain & Taylor 2003] Yun Wang, LP07, 8/16/2007

  25. Measurements of cosmic shear (WL image distortions of a few percent) left:top-hat shear variance; right: total shear correlation function. 8=1 (upper); 0.7 (lower). zm=1. First conclusive detection of cosmic shear was made in 2000. Yun Wang, LP07, 8/16/2007

  26. Cosmological parameter constraints from WL L: 8 from analysis of clusters of galaxies (red) and WL (other). [Hetterscheidt et al. (2006)] R: DE constraints from CFHTLS Deep and Wide WL survey. [Hoekstra et al. (2006)] Yun Wang, LP07, 8/16/2007

  27. Growth history of structure from WL Cosmic shear signal on fixed angular scales as a function of redshift. [Massey et al. (2007)] Yun Wang, LP07, 8/16/2007

  28. Forecasting of DE constraints from WL DUNE: 20,000 sq deg WL survey with zm=1, 1 broad red band, photo-z from ground surveys [Refregier et al. (2006)] Yun Wang, LP07, 8/16/2007

  29. DE constraints from WL depend on the accuracy of photometric redshifts Huterer et al. (2006) Yun Wang, LP07, 8/16/2007

  30. WL systematics effects • Bias in photometric redshift distribution • PSF correction • Bias in selection of the galaxy sample • Intrinsic distortion signal (intrinsic alignment of galaxies) Yun Wang, LP07, 8/16/2007

  31. Clusters as DE probe 1) Use the cluster number density and its redshift distribution, as well as cluster distribution on large scales. 2) Use clusters as standard candles by assuming a constant cluster baryon fraction, or use combined X-ray and SZ measurements for absolute distance measurements. • Large, well-defined and statistically complete samples of galaxy clusters are prerequisites. Yun Wang, LP07, 8/16/2007

  32. Clusters as DE probe • Requirements for future surveys: • selecting clusters using data from X-ray satellite with high resolution and wide sky coverage • Multi-band optical and near-IR surveys to obtain photo-z’s for clusters. • Systematic uncertainties: uncertainty in the cluster mass estimates that are derived from observed properties, such as X-ray or optical luminosities and temperature. Yun Wang, LP07, 8/16/2007

  33. Future Prospects: Recommendations bythe Dark Energy Task Force andESA-ESO Working Group on Fundamental Cosmology Yun Wang, LP07, 8/16/2007

  34. DETF Definitions • DETF figure of merit = 1/[area of error ellipse in the w0-wa plane] • DETF stages for DE probes: • Stage I: Current knowledge • Stage II: Ongoing projects • Stage III: Near-term, medium-cost projects, • Stage IV: Long-term, high-cost projects (JDEM, LST, SKA) Yun Wang, LP07, 8/16/2007

  35. DETF recommendations • Aggressive program to explore DE as fully as possible. • DE program with multiple techniques at every stage, at least one of these is a probe sensitive to the growth of cosmic structure in the form of galaxies and clusters of galaxies. • DE program in Stage III designed to achieve at least a factor of 3 gain over Stage II in the figure of merit. • DE program in Stage IV designed to achieve at least a factor of 10 gain over Stage II in the figure of merit. • Continued research and development investment to optimize JDEM, LST, and SKA to address remaining technical questions and systematic-error risks. • High priority for near-term projects to improve understanding of dominant systematic effects in DE measurements, and wherever possible, reduce them. • A coherent program of experiments designed to meet the above coals and criteria. Yun Wang, LP07, 8/16/2007

  36. ESA-ESO WG recommendations • Wide-field optical and near-IR imaging survey [WL/CL] • ESA: satellite with high resolution wide-field optical and near-IR imaging • ESO: optical multi-color photometry from the ground • ESO: large spectroscopic survey (>100,000 redshifts over ~10,000 sq deg to calibration of photo-z’s) • Secure access to an instrument with capability for massive multiplexed deep spectroscopy (several thousand simultaneous spectra over one sq deg) [BAO] • A supernova survey with multi-color imaging to extend existing samples of z=0.5-1 SNe by an order of magnitude, and improve the local sample of SNe. [SNe] • Use a European Extremely Large Telescope (ELT) to study SNe at z >1. [SNe] Yun Wang, LP07, 8/16/2007

  37. Future Dark Energy Surveys (an incomplete list) • Essence (2002-2007): 200 SNe Ia, 0.2 < z < 0.7, 3 bands, Dt ~ 2d • Supernova Legacy Survey (2003-2008): 2000 SNe Ia to z=1 • CFHT Legacy (2003-2008): 2000 SNe Ia, 100’s high z SNe, 3 bands, Dt ~ 15d • ESO VISTA (2005?-?): few hundred SNe, z < 0.5 • Pan-STARRS (2006-?): all sky WL, 100’s SNe y-1, z < 0.3, 6 bands, Dt = 10d • WiggleZ on AAT using AAOmega (2006-2009): 1000 deg2 BAO, 0.5< z < 1 • ALPACA (?): 50,000 SNe Ia per yr to z=0.8, Dt = 1d , 800 sq deg WL & BAO with photo-z • Dark Energy Survey (?): cluster at 0.1<z<1.3, 5000 sq deg WL, 2000 SNe at 0.3<z<0.8 • HETDEX (?): 200 sq deg BAO, 1.8 < z < 3. • WFMOS on Subaru (?): 2000 sq deg BAO, 0.5<z<1.3 and 2.5<z<3.5 • LSST (2012?): 0.5-1 million SNe Ia y-1, z < 0.8, > 2 bands, Dt = 4-7d; 20,000 sq deg WL & BAO with photo-z • JDEM (2017?): several competing mission concepts [ADEPT, DESTINY, JEDI, SNAP] Yun Wang, LP07, 8/16/2007

  38. How many methods should we use? • The challenge to solving the DE mystery will not be the statistics of the data obtained, but the tight control of systematic effects inherent in the data. • A combination of the three most promising methods (SNe, BAO, WL), each optimized by having its systematics minimized by design, provides the tightest control of systematics. Yun Wang, LP07, 8/16/2007

  39. Conclusions • Unraveling the nature of DE is one of the most important problems in cosmology today. Current data (SNe Ia, CMB, and LSS) are consistent with a constant X(z)at 68% confidence. However, the reconstructed X(z)still has large uncertainties at z > 0.5. • DE search methods’ checklist: 1) Supernovae as standard candles; 2) Baryon acoustic oscillations. 3) Weak lensing tomography and cosmography. • A combination of different methods is required to achieve accurate and precise constraints on the time dependence of X(z) . This will have a fundamental impact on particle physics and cosmology, and strongly recommended by DETF and ESA-ESO working group on fundamental cosmology. Yun Wang, LP07, 8/16/2007

  40. Evidence for Dark Energy Speeding up of cosmic expansion increases the distance between two galaxies (Milky Way and supernova host galaxy), which would lead to fainter than expected observed supernovae. Observed supernovae are fainter than expected, so the expansion of the universe must have accelerated. For convenience, the unknown cause for the observed acceleration of the cosmic expansion is dubbed dark energy. Yun Wang, LP07, 8/16/2007

  41. What is dark energy? Two Possibilities: (1) Unknown energy component (2) Modification of Einstein’s theory of general relativity (a.k.a. Modified Gravity) Yun Wang, LP07, 8/16/2007

  42. Spectral Signature of SNe Ia Primary feature: Si II 6355 at rest=6150Å Secondary feature: Si II 4130 dip blueshfted to 4000Å SN Ia 1999ff (z=0.455): a: Ca II H and K absorption b: Si II 4130 dip blueshfted to 4000Å c: blueward shoulder of Fe II 4555 d: Fe II 4555 and/or Mg II 4481 e: Si III 4560 i: Si II 5051 SN IIb 1993J: double peak centered just blueward of 4000Å, due to Ca II H and K absorption at 3980Å due to blueshufted H, but not similar to Ia redward of 4100Å. [Coil et al. 2000, ApJ, 544, L111] Yun Wang, LP07, 8/16/2007

  43. Understanding SN Ia Spectra Solid: Type Ia SN 1994D, 3 days before maximum brightness Dashed: a PHOENIX synthetic spectrum (Lentz, Baron, Branch, Hauschildt 2001, ApJ 557, 266) Yun Wang, LP07, 8/16/2007

  44. Weak Lensing of SNe Ia Kantowski, Vaughan, & Branch 1995 Frieman 1997 Wambsganss et al. 1997 Holz & Wald 1998 Metcalf & Silk 1999 Wang 1999 WL of SNe Ia can be modeled by a Universal Probability Distribution for Weak Lensing Magnification(Wang, Holz, & Munshi 2002) The WL systematic of SNe Ia can be removed by flux averaging (Wang 2000; Wang & Mukherjee 2003) Yun Wang, LP07, 8/16/2007

  45. Ultra Deep Supernova Survey To determine whether SNe Ia are good cosmological standard candles, we need to nail the systematic uncertainties (luminosity evolution, gravitational lensing, dust). This will require at least hundreds of SNe Ia at z>1. This can be easily accomplished by doing an ultra deep supernova survey using a dedicated telescope, which can be used for other things simultaneously (weak lensing, gamma ray burst afterglows, etc). Wang 2000a, ApJ (astro-ph/9806185) Yun Wang, LP07, 8/16/2007

  46. What is the fate of the universe?Wang & Tegmark, PRL (2004) Yun Wang, LP07, 8/16/2007

  47. Model Selection Using Bayesian Evidence Bayes theorem:P(M|D)=P(D|M)P(M)/P(D) Bayesian edidence:E=L()Pr()d :likelihood of the model given the data. Jeffreys interpretational scale of lnE between two models: lnE<1: Not worth more than a bare mention. 1<lnE<2.5: Significant. 2.5<lnE<5: Strong to very strong. 5<lnE: Decisive. SNLS (SNe)+WMAP3+SDSS(BAO): Compared to , lnE=-1.5for constant wX model lnE=-2.6for wX(a)=w0+wa(1-a) model Relative prob. of three models: 77%, 18%, 5% Liddle, Mukherjee, Parkinson, & Wang (2006) Yun Wang, LP07, 8/16/2007

  48. Wang 2006 Yun Wang, LP07, 8/16/2007

  49. ALPACA • 8m liquid mirror telescope • FOV: 2.5 deg diameter • Imaging l=0.3-1mm • 50,000 SNe Ia per yr to z=0.8, 5 bands, Dt = 1d • 800 sq deg WL & BAO with photo-z Yun Wang, LP07, 8/16/2007

  50. 8.4m (6.5m clear aperture) telescope; FOV: 3.5 deg diameter; 0.3-1mm • 106 SNe Ia y-1, z < 0.8, 6 bands, Dt = 7d • 20,000 sq deg WL & BAO with photo-z Yun Wang, LP07, 8/16/2007

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